1. Field of the Invention
The present invention relates generally to carbon or ceramic-metallic composite articles and the method of making same. More particularly the invention relates to unique carbon-silicon composite articles for use in high temperature, hostile fluid environments.
2. Discussion of the Prior Art
During recent years considerable effort has been directed toward the search for structural materials having increasingly high temperature capabilities, and superior dimensional stability, corrosion resistance, erosion resistance and tolerance to damage. In this connection substantial work has been done with metals, monolithic ceramics and carbon-graphite materials. Metals display toughness and tolerance to damage but are relatively limited in their temperature capability. Monolithic ceramics can withstand high temperatures and oxidizing environments, but are vulnerable to structural damage. Carbon-graphite composites, on the other hand, can withstand high temperature and structural damage but are subject to oxidative degredation. In view of these facts the development of new composite materials has commanded considerable attention. Since composites can combine many of the attractive features of metals while ameliorating various of the structural and degradation problems associated with carbons and ceramics, they are ideally suited for very high temperature, hostile environment applications.
In pursuit of tough high temperature composites, various types of coating processes have been suggested. These processes generally involve contacting a molten metal and a carbon body under certain conditions, generally for the purpose of producing protective coatings for the carbon body in its environment of intended use. For example, Smiley U.S. Pat. No. 3,019,128 discloses applying molten metal to a carbon body to form a metal carbide surface layer, which in combination with metal and metal oxide layers, produces a refractory and heat transfer coating desirable on rocket nozzles and the like.
Similarly, Gurinsky U.S. Pat. No. 2,910,379 discloses a process in which molten metal is applied to a carbon liquid nuclear fuel container to prevent deleterious poisoning arising from graphite reaction with nuclear fuels and fission products.
Other coating disclosures involving molten metal-carbon body contact are Steinburg U.S. Pat. No. 2,929,741, Winter U.S. Pat. No. 2,597,964 and Acheson U.S. Pat. No. 895,531.
The U.S. Pat. to Fatzer et al, No. 3,925,577 describes a process for producing coated isotropic graphite members wherein a layer of silicon is first deposited on a graphite body and then the body is heated to a temperature to cause the silicon to melt and penetrate the pores of the graphite. Finally the article is coated with a layer of silicon carbide. In Hacke U.S. Pat. No. 3,348,967 a somewhat similar process is described in which graphite or charcoal bodies are impregnated with a molten metal which will react therewith to form carbide, thereby enabling the production of a wide variety of useful products. As will become clear from the discussion which follows, these prior art patents while generally related to the present invention are clearly distinguishable therefrom.
A common thread running through many of the prior art disclosures concerning metallic coating of carbonaceous materials, and one which serves to clearly distinguish the present invention, has been the heretofore unquestioned acceptance of the basic premise that the substrate material should be isotropic and that it must have an expansion coefficient approximating that of the coating. This has been traditionally believed necessary to prevent cracking and spalling of the coating due to stresses induced by differences in the expansion coefficients when the article is subjected to thermal cycling. The U.S. Pat. No. to Howard et al 3,393,085 discusses this premise in some detail.
Another well established prior art premise was that the metallic coatings should have an adherent mechanical or chemical bond to the substrate material to assure proper load transfer and to guarantee the structural integrity of the coated composite system. As will be discussed in greater detail hereinafter, Applicant has also found this latter requirement to be not only unnecessary, but, in fact undesirable in the practice of his invention.
By way of example and to illustrate the aforementioned prior art concepts, graphite susceptions have long been used to heat silicon wafers for semiconductor processing. Because graphite is very porous and provides a means of entrapping undesirable gases and other contaminants, the susceptors are typically coated with a chemical vapor deposited (CVD) silicon carbide to render them impermeable and non-reactive. Because silicon carbide has a high coefficient of thermal expansion (CTE) of on the order of 4.5 to 5.0 in/in/.degree.C., a high expansion 4.2 in/in/.degree.C. nominal CTE graphite is used as the substrate material to assure an economic susceptor life. It is well known, however, that the actual characteristics exhibited by graphite materials can vary 10 to 15% from the nominal values described in the literature. Accordingly, given substrate characteristics, including CTE characteristics, vary widely from lot to lot. As a result of these variations in substrate expansion coefficients, the economic life of the susceptors is quite unpredictable. Compounding the problem is the fact that coating life is also highly variable and directly relates the matching of coating and substrate expansion coefficients. Thus, current practice by susceptor manufacturers is to guarantee susceptors for not more than four complete temperature cycles.
In a similar vein, various prior patents U.S. Pat. including Nos. 2,512,230 and 1,948,382 describe composites comprising coatings of silicon carbide on monolythic and composite carbonaceous substrates to provide erosion protection for the substrate as well as interlayer support and bonding between the substrate and coating. In such applications it has been uniformly taught that the thermal expansion coefficients of the substrate should approximate that of the interlayer of coating if cracking or spalling of the interlayer coating is to be prevented.
For the reasons previously discussed, great difficulty has been experienced in satisfactorily and economically manufacturing composite articles suitable for very high temperature applications in which the coefficient of thermal expansion of the coating and the substrate is matched and in which the coating satisfactorily adheres to the base material.
As will be appreciated from the discussion which follows, the process of the present invention totally overcomes the prior art problems of coating adherency and CTE matching and provides a unique CTE mismatched composite article which will maintain its dimensional stability and will effectively resist corrosion even in hostile environments at very high temperatures.
In addition to the previously identified prior art patents, applicant is familiar with the following U.S. Pat. Nos. which serve to vividly illustrate the high degree of novelty of the present invention: 3,914,508, 3,762,644, 3,759,353, 3,676,179, 3,673,051, 3,275,467, 2,614,947, 1,948,382.